chore(nodejs): refactor buffer handling

[glasstiger]

Refactor buffer handling to look more like it was before PR #42

• when flush() is called, the buffer gets compacted right after a copy is made for sending
• deprecating the copy_buffer option, we always create a copy of the sender's buffer for sending, even if calls to the sender are fully sequential
• since all async sending tasks are working on their own buffer, we can remove promise chaining